The invention relates to a digital display type tachometer for automobiles for numerically displaying a rotation speed of the engine.
Recently, a pointer type tachometer in automobiles tends to be replaced by a digital display type tachometer for numerically displaying a rotation speed of the engine.
An embodiment of the known digital display type tachometers is shown in FIG. 1, in which a counter 1 counts the number of angle pulses Sp per unit time, which pulses are generated from a crank angle sensor (not shown) depending on a rotation speed of the engine, to produce BCD (Binary Coded Decimal) signals of 1000's, 100's and 10's digits depending on the rotation speed of the engine.
The BCD signals are then latched by latch circuits 2, 3 and 4 at a predetermined timing respectively and converted into decimal character signals by decoder/driver circuits 5, 6 and 7 which character signals in turn are applied to a digital display device 8 having display sections 8a, 8b and 8c for the 1000's, 100's and 10's digits, each comprising seven segments to numerically display the rotation speed of the engine.
A display section 8d for the 1's digit is constructed to display always "zero" and is driven together with a display section 8e for "RPM" by closing an ignition switch 9.
In general, during normal driving it is superfluous to display a 10's digit of the engine rotation speed and it is sufficient to indicate the 100's and 1000's digits only.
However, in the known digital display type tachometer as shown in FIG. 1, since the display section 8a for the 10's digit is always driven, the indication on the 10's digit display section 8a varies unsteadily according to the slight variation of the engine rotation speed in driving the automobile which annoys the driver.
On the other hand, in the idling condition in which the rotation speed of the engine is between 500 and 700 rpm, it is desired to display the 10's digit in addition to the 100's digit. For example, when a heavy load such as an air conditioner and the like is loaded on the engine, the idling engine speed must be finely adjusted to a suitable speed higher than the normal idling engine speed, and for this purpose it is necessary to display a precise indication including the 10's digit.